how to setup ipsec vpn if other end 3rd party device have private ip address on external interface

[Shiva_B]

we want to setup site to site between checkpoint and 3rd party device,where in checkpoint external interface have public Ip address(112.113.211.66) but other end device(Digisol router) have private ip address(192.168.1.100) and there is another DSL router which is  present before digisol.

please help me how to setup his VPN.

[Maarten_Sjouw]

In the DSL router you will need to make sure that the device is set as a DMZ host, this way all traffic that ends up on the DSL router will be forwarded to the Digisol.
There is no way port-forwarding will work as you need protocol 50 which you cannot forward.